Innate Immunity

Innate immunity constitutes the evolutionarily ancient, non-specific arm of the immune system, providing the first line of defence against invading pathogens. Unlike adaptive immunity, innate immune mechanisms are encoded in the germline, require no prior antigen exposure, and respond within minutes to hours of pathogen encounter. These mechanisms encompass physical and chemical barriers, circulating and tissue-resident immune cells, the complement system, and a network of cytokines and mediators that coordinate inflammatory responses.

In Australia, infections related to innate immune dysfunction represent a significant clinical burden. Sepsis accounts for approximately 20,000 hospital admissions annually, with in-hospital mortality rates of 15–20%. Aboriginal and Torres Strait Islander peoples experience disproportionately higher rates of invasive pneumococcal disease, rheumatic fever, and chronic suppurative lung disease, reflecting both innate immune challenges and broader social determinants of health. Primary immunodeficiencies affecting innate pathways — including chronic granulomatous disease, complement deficiencies, and toll-like receptor signalling defects — are increasingly recognised through expanded newborn screening programmes and genetic testing available via Medicare-funded genomic sequencing (MBS item 73425).

This guideline provides a clinically focused overview of innate immune mechanisms relevant to Australian general practice, emergency medicine, and specialist immunology. It covers the recognition of innate immune deficiency, interpretation of relevant investigations, and therapeutic considerations within the Australian healthcare context.

Physical and chemical barriers represent the outermost layer of innate defence, preventing pathogen colonisation and entry into sterile tissues. These barriers are continuously active and require no immune cell activation.

Physical Barriers

Chemical Barriers

Pattern-Recognition Receptors (PRRs)

Innate immune cells and epithelial surfaces express germline-encoded PRRs that detect conserved microbial structures (PAMPs) and endogenous danger signals (DAMPs). PRR engagement triggers intracellular signalling cascades leading to cytokine production, inflammasome activation, and adaptive immune priming.

Innate immune cells are rapidly recruited to sites of infection and tissue damage, where they perform phagocytosis, degranulation, antigen presentation, and cytokine secretion. Many also bridge innate and adaptive immunity.

Neutrophils

Neutrophils are the most abundant circulating leucocytes (2.0–7.5 × 10⁹/L in adults) and the first cells recruited to sites of acute infection. They are short-lived (6–12 hours in circulation) but critical for defence against bacteria and fungi.

Monocytes, Macrophages & Dendritic Cells

Monocytes (0.2–1.0 × 10⁹/L) circulate for 1–3 days before migrating into tissues, where they differentiate into macrophages or dendritic cells. Tissue-resident macrophages (Kupffer cells, alveolar macrophages, microglia) provide sentinel surveillance.

Natural Killer (NK) Cells

NK cells (CD56⁺CD3⁻) are innate lymphocytes that provide rapid cytotoxic responses against virus-infected and tumour cells without prior sensitisation. They are controlled by the balance of activating (NKG2D, NKp46) and inhibitory (KIR, NKG2A) receptors — the "missing-self" hypothesis explains their ability to kill MHC-I-downregulated cells.

Innate Lymphoid Cells (ILCs)

ILCs are the innate counterparts of T helper subsets, residing predominantly in mucosal tissues. They lack rearranged antigen receptors and respond rapidly to epithelial-derived cytokines (IL-25, IL-33, TSLP).

Other Innate Immune Cells

• Mast cells: Tissue-resident (skin, mucosa, perivascular); degranulate on IgE crosslinking (FcεRI) or complement (C3a, C5a) — histamine, heparin, proteases, prostaglandins, leukotrienes. Central to allergic and anaphylactic responses.
• Eosinophils: 0.0–0.5 × 10⁹/L; major basic protein (MBP), eosinophil cationic protein (ECP) — helminth killing, allergic tissue damage. Eosinophilia (≥0.5 × 10⁹/L) warrants investigation for parasitic infection, allergy, eosinophilic disorders, and haematological malignancy.
• Basophils: <0.1 × 10⁹/L; circulating counterpart of mast cells; IgE-dependent activation; IL-4 source in early allergic responses.
• NKT cells: Recognise lipid antigens presented by CD1d; rapid IFN-γ and IL-4 production; bridge innate and adaptive immunity.

Primary Immunodeficiencies Affecting Innate Cells

The complement system comprises over 30 soluble and membrane-bound proteins that form an enzymatic cascade providing rapid antimicrobial defence. Complement activation occurs via three pathways — classical, lectin, and alternative — all converging at C3 convertase formation and leading to opsonisation, inflammation, and direct pathogen lysis.

Activation Pathways

Terminal Pathway & Membrane Attack Complex (MAC)

All three pathways converge to generate C3 convertase, which cleaves C3 into C3a (anaphylatoxin) and C3b (opsonin). C3 convertase combines with C3b to form C5 convertase, cleaving C5 into C5a (potent anaphylatoxin and chemotaxin) and C5b. C5b sequentially recruits C6, C7, C8, and multiple C9 molecules to form the membrane attack complex (MAC, C5b-9), creating transmembrane pores that lyse target cells.

Biological Functions of Complement

Complement Regulation

Tight regulation prevents complement-mediated host tissue damage. Key regulators include:

• C1 inhibitor (C1-INH): Serine protease inhibitor; blocks classical and lectin pathway activation. Deficiency causes hereditary angioedema (HAE) — C1-INH concentrate (Berinert®) available via special PBS authority for acute attacks.
• Factor H: Cofactor for Factor I-mediated C3b cleavage; displaces Bb from C3b. Mutations/antibodies → atypical haemolytic uraemic syndrome (aHUS) or C3 glomerulopathy.
• Factor I: Serine protease that cleaves C3b and C4b (requires cofactors: Factor H, MCP, C4BP).
• Decay-accelerating factor (DAF/CD55): Accelerates decay of C3/C5 convertases. Deficient in paroxysmal nocturnal haemoglobinuria (PNH) — treated with eculizumab (Soliris®), PBS Authority Required.
• Membrane cofactor protein (MCP/CD46): Cofactor for Factor I; mutations predispose to aHUS.
• CD59 (protectin): Prevents C9 polymerisation and MAC insertion. Deficient in PNH.

Complement Deficiencies & Clinical Syndromes

Investigations — Complement Assessment

Cytokines are small signalling proteins that coordinate the innate immune response, regulate inflammation, and bridge to adaptive immunity. They act in autocrine, paracrine, and endocrine fashions, forming complex networks with synergistic, antagonistic, and pleiotropic effects. Dysregulated cytokine production underlies sepsis, cytokine release syndrome (CRS), autoinflammatory diseases, and chronic inflammatory conditions.

Pro-inflammatory Cytokines

Anti-inflammatory Cytokines

Chemokines

Chemokines are small chemoattractant cytokines that direct leucocyte migration. They are classified by cysteine residue arrangement (CXC, CC, CX3C, C).

Other Key Mediators of Innate Immunity

• Prostaglandins & Leukotrienes: Arachidonic acid metabolites (via COX-1/2 and 5-LOX pathways). PGE₂ causes vasodilation, fever, pain sensitisation. LTB₄ is a potent neutrophil chemoattractant. Cysteinyl leukotrienes (LTC₄, LTD₄, LTE₄) cause bronchoconstriction and mucus secretion — target of montelukast (Singulair®, PBS General Benefit).
• Histamine: Released from mast cells and basophils; H1 receptor activation causes vasodilation, increased permeability, pruritus, bronchoconstriction. H1 antihistamines (cetirizine, fexofenadine — PBS General Benefit) are first-line for allergic symptoms.
• Nitric Oxide (NO): Produced by inducible NOS (iNOS/NOS2) in macrophages; antimicrobial via reactive nitrogen species; also vasodilator contributing to septic shock.
• Reactive Oxygen Species (ROS): Superoxide (O₂⁻), hydrogen peroxide (H₂O₂), hypochlorous acid (HOCl) — generated by NADPH oxidase in phagocytes; microbicidal but cause tissue damage in chronic granulomatous inflammation.
• DAMPs (alarmins): HMGB1, ATP, uric acid crystals, heat shock proteins, mitochondrial DNA, IL-33, IL-1α — released from damaged/necrotic cells; activate PRRs and sterile inflammation. Relevant to trauma, ischaemia-reperfusion injury, and gout (monosodium urate crystals activate NLRP3 inflammasome).

Cytokine Storm & Clinical Syndromes

Recognition of innate immune deficiency in primary care depends on pattern recognition of recurrent, severe, or unusual infections. The Australasian Society of Clinical Immunology and Allergy (ASCIA) provides referral guidelines.

Red Flags for Referral to Immunology

• ≥2 systemic bacterial infections in 12 months (septicaemia, deep abscesses, osteomyelitis, pneumonia requiring hospitalisation)
• Infections with unusual organisms (Aspergillus, Nocardia, Serratia, atypical mycobacteria, Pneumocystis jirovecii) in an immunocompetent-appearing host
• Failure to thrive with recurrent infections in infants
• Recurrent meningococcal disease (consider terminal complement deficiency)
• Family history of primary immunodeficiency or consanguinity
• Persistent oral candidiasis or dermatophytosis beyond childhood (consider STAT1 GOF, CARD9 deficiency)
• Severe viral infections (disseminated HSV/VZV, fatal EBV) — consider inborn errors of type I IFN immunity

Primary Care Investigations (Before Specialist Referral)

1. 1. Murphy K, Weaver C. Janeway's Immunobiology. 10th ed. New York: Garland Science; 2022.
2. 2. Abbas AK, Lichtman AH, Pillai S. Cellular and Molecular Immunology. 10th ed. Philadelphia: Elsevier; 2022.
3. 3. Australasian Society of Clinical Immunology and Allergy (ASCIA). ASCIA Guide to Investigation and Management of Primary Immunodeficiency. Sydney: ASCIA; 2024. Available from: https://www.allergy.org.au
4. 4. Tangye SG, Bucciol G, Meyts I. Human inborn errors of immunity: 2024 update on the classification from the International Union of Immunological Societies Expert Committee. J Clin Immunol. 2024;44(1):22–68.
5. 5. Mastaglia FL. Innate immunity and the role of pattern recognition receptors. Aust Prescr. 2021;44(4):118–122.
6. 6. Australian Commission on Safety and Quality in Health Care (ACSQHC). Sepsis Clinical Care Standard. Sydney: ACSQHC; 2023. Available from: https://www.safetyandquality.gov.au
7. 7. Merle NS, Church SE, Fremeaux-Bacchi V, Roumenina LT. Complement system part I — molecular mechanisms of activation and regulation. Front Immunol. 2015;6:262.
8. 8. Australian Institute of Health and Welfare (AIHW). Aboriginal and Torres Strait Islander Health Performance Framework 2023 Summary Report. Canberra: AIHW; 2023.
9. 9. RHDAustralia (RHD Australia, ARF/RHD writing group). The 2020 Australian guideline for prevention, diagnosis and management of acute rheumatic fever and rheumatic heart disease. 3rd ed. Darwin: Menzies School of Health Research; 2020.
10. 10. Dinn RL, Bhojwani R, Engel M, et al. Inborn errors of immunity in Australia: a national approach to newborn screening. J Clin Immunol. 2023;43(6):1068–1078.
11. 11. Bowen AC, Mahé A, Hay RJ, et al. The global epidemiology of impetigo: a systematic review of the population prevalence of impetigo and pyoderma. PLoS One. 2015;10(8):e0136789.
12. 12. The RECOVERY Collaborative Group. Tocilizumab in patients admitted to hospital with COVID-19 (RECOVERY): a randomised, controlled, open-label, platform trial. Lancet. 2021;397(10285):1637–1645.
13. 13. Fajgenbaum DC, June CH. Cytokine storm. N Engl J Med. 2020;383(23):2255–2273.
14. 14. Pharmaceutical Benefits Scheme (PBS). Australian Government Department of Health and Aged Care. Available from: https://www.pbs.gov.au. Accessed 2024.